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Audio 101 Part Four: Amplifier Ins and Outs

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Written by Colin Smith

Created: 01 December 2010

Amplifiers work by amplifying input signals -- of course. But did you know that what leaves an amplifier is not what entered it? That’s because amplifiers aren’t linear, in the sense that they don’t represent a single progression, as you would get when using a lens to zoom in on something small. Instead, an amplifier duplicates the input signal as a much more powerful version of itself.

How amplifiers do this involves the movement of electrons from one side of a transistor or vacuum tube (the emitter) to the other (the collector) when certain electrical conditions are met. For reasons known only to the likes of Nelson Pass, this process boosts the voltage on the collector side of the transistor, and this is what drives loudspeakers.

That’s all well and fine, but what sorts of things do we need to consider when buying an amplifier?

First, let’s discuss some amplifier terminology. Then I want you to forget it all and focus on what’s important.

When you go into your local audio retailer, you may be confronted by salespeople who want to prove their importance by spouting all sorts of arcane specifications for Amplifier A that make sense only to people like . . . well, like me. But you don’t want to be like me. If you were like me, you’d be sitting in front of your iMac listening to Brahms and trying to figure out how to explain amplifier physics -- something that you "get" but have never had to explain to someone else.

OK, here come the terms: Power output refers to how many watts an amplifier produces -- a number that, in some amplifiers’ published specifications, should be taken with a grain of salt. If, say, Sony says its $399 receiver puts out 100Wpc, they’re talking about a theoretical maximum using a measurement system that no one likes and that many abuse. That 100W rating therefore tells you almost nothing. On the other hand, if an amplifier company such as Bryston says that its 4B SST²model puts out 300W, you can take that to the bank, secure in the knowledge that those 300W are not toxic assets.

An amplifier’s gain is how much an amplifier boosts the signal; that is, it’s the difference between the strength of the signal at the amp’s input and the signal strength at its output, measured in decibels (dB). Going back to that 4B SST²: given a 1V input signal, it offers 29dB of gain. In other words, that means it’s amplifying the input signal about 32 times -- quite a leap. That’d be like a turbocharger turning a Ford Focus into a locomotive.

Bandwidth, or power bandwidth, is the frequency range over which the amp operates. In the case of the 4B SST², its bandwidth is stated as being "less than 5Hz to over 100kHz," or from far below to way above the limits of human hearing.

Input impedance is the resistance (aka load) "seen" by the device connected to an amplifier’s input. An amplifier’s input impedance should always be many times higher than the output impedance of the device it’s connected to, to reduce any parasitic noise that might otherwise flow from a source component into an amplifier, and ensure that the strength of the source signal isn’t diminished by the amplifier’s circuitry.

An amplifier’s damping factor is its ability to prevent the drive-unit(s) of the speaker it’s connected to from making unwanted movements near the driver’s resonant frequency. Everything has a natural resonant frequency: You do, I do, so does my dog and my electric toothbrush. If you look at the specs for, say, the Scan-Speak Revelator 8" woofer, you find that its resonant frequency (fs) is 20.5Hz. In other words, if you were to run signals through that woofer near 20.5Hz, you’d find that the woofer wants to vibrate with the signal it’s being fed, and it wants to vibrate sympathetically with that signal because its natural resonant frequency is 20.5Hz. But it’s the job of an amplifier to make sure that the speaker doesn’t vibrate sympathetically. The higher the damping factor, the better the control. The 4B SST²’s damping factor is stated as being "over 500 at 20Hz."

Distortionis great if you’re a heavy-metal fan, but otherwise it’s not something we like in the reproduction of music. There are two primary kinds of distortion that amp makers refer to on their spec sheets: total harmonic distortion (THD) and intermodal distortion (IM). What’s important here is that you want to see exceedingly low numbers for both. Some say that a THD of less than 1% is inaudible. That must mean that the Bryston 4B SST²’s THD+N (I’ll get to the N) of 0.005% is so inaudible that you’d need a very expensive measuring device to even know it’s there. The N stands for noise, as in anything that isn’t audio signal but that leaches its way into the signal path. The 4B SST2’s level of noise is -110dB at 29dB gain. Put another way, the noise is as low as the dynamic capability of the CD is high, give or take.

The fact is that any good modern amplifier will have low distortion, a more than adequate damping factor, and enough gain to drive speakers with relative ease. Ah-ha, you say, but how do you know you’ve got a good amplifier?

The answer, again, is simple. I’ve said this before, about speakers, and I’ll say it again for amps: If you stick with a company that’s known for making amps, chances are high you’ll get a very good one. Who makes good amps? NAD, Anthem, Audio Research, Ayre Acoustics, Bryston, Bel Canto, Blue Circle, Boulder Amplifiers (better move further into the alphabet) . . . Hegel, Threshold, and Zandén, among others. The list is long, and you find out who’s on it by reading our reviews. As always, listen for yourself and make up your own mind.

OK, there’s a bit more to choosing an amp than that. As I mentioned in my speaker lesson, the sensitivity of your speakers may demand a more powerful amplifier to get performance that satisfies you. If your speakers have a low efficiency rating (circa 85dB), you’ll want a more powerful amplifier. On the contrary, very efficient speakers (90dB or higher) need little power to sound their best. Electrostatic speakers are power-hungry, so you’ll want to match them up with a beefy amp -- to be on the safe side, a couple of hundred watts into 8 ohms.

If you’ve made it this far, you might be wondering when I’m getting to the promised preamplifier section. Good news -- it’s right here.

All amplifiers have or need a preamp of some sort. Sometimes it’s an external box, sometimes it’s an internal gain stage, and sometimes it’s a full preamp that shares the same chassis with the power amp, in which case it’s called an integrated amplifier.

A preamplifier goes in the signal path after a source component and, as its name suggests, before the power amplifier. It does amplify the signal itself, but only a little bit. Basically, a preamp provides an intermediate stage between the low-level signal output of a source device and the relatively high level an amplifier prefers at its inputs. For completeness, a phono preamplifier will comprise a number of gain stages to boost the very-low-level electrical signals generated by a phono cartridge into something that even a preamp can work with.

Preamps are also sometimes called control amplifiers because, in addition to providing a little bit of gain, preamps offer such conveniences as volume controls and source selectors. In a volume control, the input signal is fed through a variable resistive path: when more resistance is applied to the signal, the signal is attenuated and the amplifier is fed a weaker signal, which results in less output from the speakers. Conversely, you increase the volume by decreasing the resistance; the amp gets more signal and the music gets louder.

Should you buy your preamp and amp from the same manufacturer? They prefer it if you do, but it’s not necessary. Companies like Bryston and Simaudio go to a lot of trouble to ensure that their amps and preamps have precisely the same sonic characteristics, and that can be important. But mixing and matching different brands can also result in excellent sound. The trick is to experiment -- or do what I do and buy an integrated amp. It’s so much simpler, and fewer cables, too.

We’ve covered speakers, power amplifiers, and preamplifiers. Next up will be sources. But if you’re sweet on CD players, you might not like what’s coming in Part Five.